Liquid crystal display (LCD) plays an important role in the photonics industry and has explosive growth in recent years. Demands on the LCD increase rapidly with spreading applications in various electronic products. Electronic products equipped with a LCD device include: mobile phones, personal digital assistance (PDA), notebook computers, liquid crystal monitors, and recently the LCD-TV, etc.
Polyimide (PI) is widely used as a protective material or an insulation material in the electronic field due to its good properties, e.g. high mechanical strength, high thermal resistance, and solvent resistance. Polyimide is also used to form an alignment film for LCD, which can provide a uniform and stable alignment effect to the liquid crystal molecules.
A stable and high pretilt angle of the LCD alignment film is difficult to be obtained by a rubbing process. In order to solve this problem, JP-A-62-297819 discloses an alignment film produced by mixing a long chain aliphatic compound; and JP-A-64-25126 discloses an alignment film comprising an aliphatic diamines. The abovementioned patents provide a feasible method of introducing an aliphatic compound to generate the pretilt angle, but lead to two problems which will be discussed in the following.
An alignment film made of polyimide introduced with long aliphatic groups has the problems of a poor thermal stability and an unstable pretilt angle. When the alignment film with aliphatic groups is subjected to an isotropic temperature of a liquid crystal, the pretilt angle thereof will reduce; although the pretilt angle will promptly increase after injection of the liquid crystal. Temperature of 200˜300° C. is normally adapted to cure the alignment film. Due to its poor thermal stability, however, an alignment film with long aliphatic groups will generate problems such as a reduced or non-uniform pretilt angle after the high temperature baking treatment.
In addition, when long aliphatic groups are introduced into a polyimide, the surface hydrophobia of the resulting alignment film will cause a reduced wettability to liquid crystal. If more aliphatic groups are introduced in order to increase the pretilt angle, the wettability thereof to liquid crystal will be further reduced to generate a poor display quality of the LCD device.
It is desirable to develop an alignment film having a high pretilt angle, a good thermal stability, and a good wettability to liquid crystal.
U.S. Pat. No. 5,783,656 discloses a polyamic acid introduced with a novel long alkyl structure; U.S. Pat. No. 6,111,059 discloses a method for introducing a novel long alkyl structure into a diamino monomer; and Japanese Patent Laid-Open 2000-63515A discloses a cyclic aliphatic dianhydride introduced with a novel long alkyl structure.
Furthermore, U.S. Pat. No. 5,954,999 discloses a method for generating the pretilt angle by introducing a long alkyl into the soluble polyimide moiety. The drawbacks of this method include: the long alkyl chain group rotates easily and does not generate enough steric hindrance, causes the unstable pretilt angle properties of liquid crystals and poor alignment uniformity.
Therefore, it is desirable to provide a material fulfilling the requirements for vertical aligning (VA mode) LCD with the properties of: a high pretilt angle, a controllable vertical alignment of liquid crystal, forming no scratch or flaking, easy in synthesis, a high transparency, a uniform alignment, and a good thermal stability.